Is it possible to use a slider toggle from FK to IK, in a armature?
For the former, it was never version specific and can still be done with later versions, people have desisted from doing so since there are several more efficient ways to do so.
The method I personally use and I think is most people use is a blend of constraints, drivers and a little scripting to create GUI components in the sidebars to toggle from FK &lt;-&gt; IK. I cannot say exactly when this became available but it is a core part of the application.